The desirability of higher speed and higher density in integrated circuits has resulted in multi-level conductor layers. Typically, in multi-level conductor integrated circuits, the lower layer or layers are polysilicon with the last layer being aluminum. Due to the heating involved during typical processing, aluminum was used as the last conductor layer only. Techniques have been developed using polyimide so that multi-level metal layers are feasible. The polyimide is used as insulation between metal layers. The polyimide insulating layer provides many of the benefits of insulating layers used between polysilicon and metal layers but without requiring nearly as much heat.
For an upper conducting layer to be useful, of course then it must be able to make contact with a lower conducting layer. In order for contact to be made, holes through the polyimide must be made. The holes are typically called "vias". A via is made in a location where electrical contact is to be made between two metal layers. The lower metal layer has a layer of polyimide overlying it through which vias are made at the desired locations. After the vias are made, the next layer of metal is deposited into the via, as well as on the polyimide, to make contact with the lower metal layer.
Initially, wet etching was suggested by the manufacturers of polyimides as the best way to produce vias. The wet etch methods produced vias quite reliably which had nearly optimum slopes. This worked very well until vias sizes were required to be less than 5 microns in diameter. The problems were that the vias were no longer etched clear in a reliable manner, and when the wafer was etched long enough to clear the vias, larger openings in the circuit were severely undercut. The next approach was to use a dry etch of the vias. The wafers with a photoresist mask over polyimide were dry etched in a plasma which contained oxygen. The oxygen plasma etched polyimide well, but it also etched the photoresist at the same or even faster rate. Consequently, the photoresist would often break down in areas where it was thin before all of the polyimide had completed etching in all parts of the circuit. To overcome this, a mask which did not erode under the plasma was used between the resist and the polyimide. This mask was etched at the location of the via, then the polyimide was reactive ion etched with an oxygen plasma. This allowed etching past the time which it took to clear the photoresist, which made it possible to overetch enough to clear the polyimide out of all parts of the circuit while forming the vias. The problem with this method was that the vias obtained had nearly vertical walls. The resulting poor metal step coverage causes a reduction in yield and consequently an increase in cost.
In an effort to produce more tolerable via profiles, relatively high pressure oxygen plasmas have been used to produce isotropic profiles, or a combination of high pressure oxygen plasma followed by a low pressure reactive ion etch to produce combination profiles. These procedures prove to be difficult to reproduce and require several different pieces of equipment used in sequence to run them, as well as giving less than ideal via profiles.